1. Field of the Invention
This invention relates to an ink jet printer head and an ink jet printer, and more particularly to an improvement in a so-called "on-demand" type ink jet printer that can print a half tone.
2. Description of the Related Art
Conventionally, the on-demand type ink jet printer takes the form of a printer for discharging liquid ink drop from the nozzle in accordance with a recording signal to print the material to be printed on a recording medium such as paper or film. Such a type of printer is becoming more and more widely used simply because the size of the printer can be reduced and the cost thereof can be reduced as well.
At the same time, particularly in offices, document preparation using a computer often referred to as desktop publishing has become very popular. Quite recently, requirements have grown more and more strong not only for outputting characters and figures but also for outputting natural color images such as photographs together with characters and figures. For printing such high-quality natural images, a half tone representation is very important.
In such on-demand ink jet printers, methods have become very popular which uses a piezo element, or which uses a heating element. The method which uses the piezo element involves applying pressure to the ink through the distortion of the piezo element to discharge the ink from the nozzle. The method which uses the heating element involves localized heating and boiling the ink with the heating element to discharge the ink with the pressure of the bubbles thus generated.
In addition, ink jet printers are available which vary a voltage and a pulse width to be given to the piezo element or the heating element and control a liquid ink drop that is discharged to render variable the diameter of printing dots and thus provide a gradation representation. Ink jet printers are also available which provide gradation representation in matrixes using the dithering method by constituting one pixel with a matrix comprising, for example, 4.times.4 dots without changing the dot diameter.
However, as described above, in the on-demand ink jet printer, the method for changing a voltage and pulse width applied to the piezo element or the heating element has the following drawbacks: when the voltage and the pulse width applied to the piezo element and the heating element are excessively lowered, ink cannot be discharged, meaning that the minimum liquid drop diameter is limited. Consequently, the number of gradation levels that can be represented is low. Low-density representation in particular cannot be done. The method is thus unsatisfactory, practically speaking for printing out natural images.
When one pixel is constituted, for example, of 4.times.4 matrixes with the method for gradation representation using the dithering method, 17 gradation levels can be represented. However, the material to be printed is printed in the same dot density as the above method, the resolution will deteriorate only to one fourth, so that the roughness of the printed characters becomes apparent. In such a case, the method is unsatisfactory, practically speaking, for printing out natural images.
To solve such problems, an ink jet printer, which discharges a mixed liquid obtained by quantifying and mixing transparent solvent and ink to perform printing, has been provided. In this type of ink jet printer, one of the transparent liquid solvent and the ink, for example, ink is quantified to obtain a desired gradation, the quantified ink is mixed with other liquid, for example, transparent solvent, and the mixed liquid is discharged as a fixed amount to perform printing. That is, printing is performed by in-dot density gradation.
As an ink jet printer for printing by using mixed liquid that ink and transparent solvent are mixed, an ink jet printer has been provided in which ink and transparent solvent is quantified and mixed by utilizing an electrical permeation technique (Japanese Patent Laid Open No. 201024/1993 (U.S. Pat. No. 961,982)). Here, the electrical permeation is a phenomenon wherein electrolyte solution moves through a porous barrier membrane, when a porous barrier membrane is provided to partition a vessel filled with electrolyte solution into two, for example, into right and left, chambers and electrode plates are put into respective partitioned electrolyte solution to apply voltage.
Since the permeation amount (movement amount) of the electrolyte solution is proportional to the voltage when electrical permeation is used, the relatively accurate quantifying and mixing can be performed. However, because the frequency response of the electrical permeation is lower than, for example, a piezo element or a heat generating element, it has been difficult to realize high speed printing. Moreover, there has been a problem that because the electrolyte solution is used in the electrical permeation, if water is used as transparent solvent, it is electrolyzed and bubbles are generated.